Are Lithospheres Forever?

Abstract

The Earth's lithosphere has a conductive (or advective) geothermal profile in contrast to the convecting asthenosphere with its adiabatic thermal gradient. The thickness of the sub-continental lithosphere (SCLM) varies with tectonothermal age: Archaean and Proterozoic SCLM is relatively cold, geochemically depleted and thick, while Phanerozoic SCLM is thinner, hotter and fertile. The SCLM is a palimpsest that carries a geochemical, thermal and chronological record of large-scale tectonic events that have shaped the Earth's crust. Development of the 4-D Lithosphere Mapping methodology (O'Reilly and Griffin, 1996) has allowed the construction of realistic geological sections of the SCLM in a wide variety of tectonic settings. Mantle-derived xenoliths and garnet and chromite xenocrysts from volcanics (eg basalts, lamproites, kimberlites) provide samples of the lithospheric mantle at the time of eruption. Where sufficient xenoliths and/or xenocrysts of appropriate composition are available, we can determine the palaeogeotherm, the depth to the crust-mantle boundary, the detailed distribution of rock types with depth within the SCLM, the spatial distribution of fluid-related (metasomatic and anatectic) processes and the depth to the lithosphere-asthenosphere (LAB) boundary, within the tectosphere. Volcanic episodes of different ages in one region provide this information for different time-slices corresponding to ages of the volcanism, while geophysical data (seismic, gravity, magnetic, thermal) can be used to extend the geologically-derived profiles laterally or to interpret lithospheric domains with geophysical signatures that can be matched with geologically mapped sections.